The present invention relates to resins obtained by Diels-Alder polymerization, and more particularly, to high temperature resistant matrix resins obtained by Diels-Alder polymerization of benzocyclobutene-maleimide compounds.
The Diels-Alder reaction is a cycloaddition reaction in which an unsaturated group, which is a dienophile, combines with a 1,3-diene to form a six membered ring. The Diels-Alder reaction appears to be favored by the presence of the diene's electron-yielding groups, and the dienophile's electron-attracting groups.
Although the Diels-Alder reaction is used extensively in organic chemistry, it is less commonly employed in polymer chemistry. Yet, it can be quite advantageous. For example, at high temperatures, it is expected that Diels-Alder adducts with appropriate activating groups (unsaturated and conjugated groups) undergo conversion to other products, such as aromatic rings, before the temperature required for the reverse reaction or degradation process is reached; consequently, such Diels-Alder polymers exhibit high thermal stability.
In most Diels-Alder polymerizations, a bis-diene reacts with a bis-dienophile. For example, in W. J. Bailey et al, "Polymeric Diels-Alder Reactions," J. Org. Chem. 27, 3295(1962), 2-vinylbutadiene, a bi-functional diene, is reacted with benzoquinone, a dienophile. J. K. Stille, "Cycloaddition Polymerization," Die Makromolekulare Chemie 154, 49(1972), teaches that cyclopentadienones undergo a variety of Diels-Alder reactions depending on the ring substitution, dienophile, and reaction conditions. To obtain a monoadduct, cyclopentadienone is employed with an acetylenic dienophile to obtain an aromatic product. Additionally, R. T. Kohl et al, "Diels-Alder Reactions of Phenyl-Substituted 2-Pyrones: Direction of Addition with Phenylacetylene," Macromolecules 11, 340(1978) shows the Diels-Alder reactions of substituted acetylenes with 2-pyrones. J. N. Braham et al, "Polyphenylenes via Bis(2-pyrones) and Diethynylbenzenes, "The Effect of m- and p-Phenylene Units in the Chain," Macromolecules 11,343(1978) shows the Diels-Alder 4+2 cycloaddition reaction of bis(2-pyrone) monomers with diethynylbenzenes.
In some Diels-Alder polymerizations, the same molecule contains both the diene and the dienophile moiety. One class of monomer is capable of functioning as both a diene and dienophile. Cyclopentadiene and 2-vinylbutadiene are two examples. In another class of monomer, the diene and dienophile are different. Meek and Argabright, J. Org. Chem. 22, 1708(1957) prepared 6-[p-(p-maleimidobenzoyloxy)phenyl]-1,2,3,4-tetrachlorofulvene which contains a maleimido group as a dienophile and a perchlorofulvene group as a diene. W. J. Bailey, "Diels-Alder Polymerization, " Step-Growth Polymerization, Marcel Dekker, New York, 1972, stresses that this type of polymerization presents considerable difficulty.
Benzocyclobutene functions very well in a Diels-Alder reaction. As taught by W. Oppolzer, Synthesis 793(1978), under appropriate thermal conditions, the benzocyclobutene unit undergoes an electrocyclic ring opening to form the more reactive o-xylylene functionality. O-xylylene is a powerful diene and, engages in a Diels-Alder reaction in the presence of a suitable dienophile. See Boekelheide, Accounts Chem. Res. 13, 65(1980).
An example of a suitable dienophile is a maleimide. Maleimides are well-known as possessing strong dienophilicity. The dienophilic site, i.e., the carbon-carbon double bond, is not subjected to the substituent effect imposed by the rest of the structure. Thus, a maleimide engages in a Diels-Alder polymerization in the presence of a suitable diene such as o-xylylene.
Bis-benzocyclobutenes and polymers derived therefrom are disclosed in U.S. Pat. No. 4,540,763. The bis-benzocyclobutenes are connected by direct bond or a bridging member such as a cyclic imido group. In general, the polymers are obtained by addition polymerization wherein the fused cyclobutene rings undergo thermal transformation to o-xylylene moieties which can react with one another.